The increase in the price of copper (Cu) and in the value of alloy metals such as in particular nickel (Ni), cobalt (Co) and molybdenum (Mo) has led copper smelters to develop complementary steps for treating slag prior to dumping.
This is because processes for smelting Cu from ore concentrates generally give rise to copper matte, on the one hand, which is refined, and on the other hand to slag or cinder resembling gangue, composed primarily of silica, iron oxides, and other “non-reducible” oxides (alumina, magnesium, lime). This slag contains low contents of copper and other reducible metals, such as molybdenum, nickel, cobalt etc., which have hitherto been only partially recovered, either by a physical method, namely very fine grinding of the slag and separation of the phases containing copper by flotation, or by a slag reduction treatment in an electric furnace.
In practice, copper smelting plants have always left behind and dumped a low-grade fraction still containing 0.5 to 2% of copper and contents of the order of 0.5% of Mo, Ni or Co which vary as a function of the mineralogical nature of the concentrate.
A typical analysis of slag originating from smelters is presented in Table 1 below:
TABLE 1Cu smelter slagElement/compoundUnitContentTotal Cu%0.7~1.6Fe%44~47Mo%  0~0.6Zn%1~5S%0.2~0.8SiO2%24~29Fe3O4% 4~11CaO%1~2Al2O3%2~3
Slag of a comparable composition (also containing Cu) is obtained from metallurgical processing of lead, after the steps of extraction by volatilisation (“fuming”) of the volatile metals, mainly Pb and Zn.
The composition of this slag is summarised in Table 2 below:
TABLE 2Pb “fuming” slagElement/compoundUnitContentTotal Cu%1~2Fe (as FeO)%20~35SiO2%30~40CaO%15~20Al2O3%3~7Zn%0~1Pb%<0.1
Furthermore, electrochemical smelting of copper (anode furnaces) gives rise to slag known as copper-iron slag, of the following (indicative) composition (Table 3):
TABLE 3Cu—Fe slagElement/compoundUnitContentTotal Cu% 5~10Fe%25~35Sn%1~2Ni%0~1Cr%0~3S%0.1~0.2SiO2%10~30CaO% 5~15Al2O3%2~6MgO%2~4Zn% 0~10Pb%0~2
In known recovery processes, the recovery step is performed in a treatment or purifying (“slag cleaning”) furnace, the most recent version of which is an electric furnace of the SAF (“Submerged Arc Furnace”) furnace type, in which heating does not in fact take place by electrical arc but instead by current conduction (Joule effect) in the slag; this is why this furnace is sometimes known as a “slag resistance” furnace.
The technology and operation of this type of furnace is described in the article “Current status and new trends in copper slag cleaning” in the publication “COPPER 2003-COBRE 2003”.
In industrial practice, these furnaces process several types of smelting coproducts, but mainly converter slags which are liquid and contain 5 to 10% Cu (mainly as the oxide Cu2O and the sulfide Cu2S), and, in small proportions, “copper scraps” which are cold and therefore solid and contain 30 to 40% Cu. The other dominant compounds are iron (as oxides FeO and Fe3O4) and silica SiO2.
The operation is performed at a temperature of the order of 1250° C. and involves reducing Cu oxides using gaseous CO or metallic iron, themselves formed through the addition of carbon to the slag.
The products obtained are a Cu—Fe—S matte containing 50 to 70% of copper, and thus a considerable proportion of iron, and a residual slag of the composition stated above (“Cu smelter slag”).
This result is unsatisfactory for 2 reasons:                on the one hand, the copper matte is greatly diluted by iron and sulfur, and        on the other hand, the final slag still contains significant contents of Cu, Mo and Zn, which will not therefore be recovered.        
A publication by MINTEK, entitled “Recovery of cobalt, nickel, and copper from slags, using DC-arc furnace technology” presents the results of tests carried out on various direct current-powered pilot arc furnaces (60 kW, 150 kW, 1-3 MW) involving fusion of slag from reverberatory furnaces containing 1 to 3% Cu, 0 to 4% Ni, 0 to 1% Co.
The tests were carried out at temperatures of between 1300 and 1600° C. and reduction was performed using carbon as the primary reducing agent.
The alloys obtained comprised 40% to 80% Fe, and the main findings obtained from the tests were as follows:                cobalt and copper are more difficult to recover than nickel,        recovery yields for Cu and Co are principally a function of the proportion of iron in the poured metal and of the duration of the operation, with durations of the order of 2 h making it possible to minimise residual contents of Cu, Ni and Co in the slag, and        for iron contents of 50% in the metal, the residual contents observed are of the order of 0.5% Cu, and of 0.1% for Ni and Co, these figures resulting from initial contents 5 to 10 times higher for Ni and Cu, which confirms that Ni is recovered with the best yield.        
The study further cites                the possibility of concentrating the Cu and Co alloy by injection of oxygen, making it possible to reduce the content of Fe to 25%, and        the possibility of improving the recovery yields of the valuable elements by application of “mild stirring”, for which no figures are given.        
The authors explain that plasma arc heating makes it possible to obtain better recovery yields than in a slag resistance furnace (mentioned above), because the chemical composition of the slag may be adjusted independently of the slag resistance feature (which requires elevated Fe contents); however, the comparison is limited to the AC (alternating current) electric furnace variant of the SAF type relative to the DC-type plasma arc furnace.
Finally, they mention prior publications which have demonstrated that the addition of lime (CaO) accelerates reduction kinetics and makes it possible to obtain good recovery yields after as little as 1 h of operation.
In 1994, MINTEK filed a patent based on this principle of reduction using carbon in a DC (direct current) arc furnace.
It should be noted that there are a number of old patents (1972 to 1988) which propose Cu “smelter” slag reduction processes using an electric furnace, under very specific conditions:                a 1972 patent (DE 2122169, Vereinigte Deutsche Metallwerke) using CaSi fines and very particular temperature conditions (>1700° C.),        a 1976 patent (GB 1458269, Kennecott Copper) using as reducing agent iron sulfide at 1200-1300° C. or a carbon-containing reducing agent, introduced with intense mechanical stirring in order ultimately to form a matte, and        a 1988 patent (US 4737186, Outokumpu) relating to the reduction of a Pb, Cu and/or Ni slag using carbon dust, but on a very thin metal bath (thickness less than 20 mm), which involves the SAF-type mode of operation (resistive slag).        
Consequently, none of the hitherto proposed solutions allows satisfactory recovery of valuable metals, either with regard to the nature of the raw materials which may be used and the yield of the recovered metals, or with regard to the technical and economic aspects of the process itself.